Technical Field
The present invention relates to an intermittent failure diagnostic system to diagnose a failure (abnormality, fault) of an electrical apparatus, an electrical equipment and so on such as an electric power steering apparatus or the like to apply an assist torque to a steering system of a vehicle by a rotational torque of a motor, and in particular to an intermittent failure diagnostic system that detects an intermittent failure by applying a statistics way (a normal distribution probability, an upper-limit side probability and a lower-limit side probability), suppresses an unprepared system down and improves a reliability and to an electric power steering apparatus provided with the same.
Background Art
As an electrical apparatus and an electrical equipment and so on (hereinafter merely refer to “electrical apparatus”) comprising an electrical machinery and apparatus and electrical elements and so on, there has been an electric power steering apparatus (EPS) that energizes a steering system of a vehicle by using a rotational torque of a motor as an assist torque, applies an assist force of the motor as the assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate the assist torque (steering assist torque), such a conventional electric power steering apparatus performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a current command value and a motor current becomes small, and the adjustment of the voltage applied to the motor is generally performed by an adjustment of a duty ratio of a pulse width modulation (PWM) control.
A general constitution of a conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft, handle shaft) 2 connected to a steering wheel (handle) 1, is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack and pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. Further, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque Tr of the steering wheel 1, and a motor 20 for assisting the steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist command based on the steering torque Tr detected by the torque sensor 10 and a vehicle speed Vel detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 based on a voltage command value E obtained by performing a compensation and so on with respect to the calculated current command value. Further, it is also possible to receive the vehicle speed Vel from a controller area network (CAN).
The control unit 30 mainly comprises a CPU (or an MPU or an MCU), and general functions performed by programs within the CPU are shown in FIG. 2.
Functions and operations of the control unit 30 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque Tr detected by the torque sensor 10 and the vehicle speed Vel detected by the vehicle speed sensor 12 are inputted into a current command value calculating section 31. The current command value calculating section 31 calculates a current command value Iref1 based on the steering torque Tr and the vehicle speed Vel. The calculated current command value Iref1 is inputted into a subtracting section 33 through a maximum output limiting section 32 to limit the maximum value. The subtracting section 33 obtains a current deviation Iref3 (=Iref2−i) of the current command value Iref2 of which maximum value is limited and a motor current i being fed back, the current deviation Iref3 is PI-controlled at a PI-control section 34. The PI-controlled voltage command value E is inputted into a PWM-control section 35 and is calculated duty ratios, and the motor 20 is PWM-driven through a motor driving circuit 36 such as an inverter with a PWM-signal PS. The motor current value i of the motor 20 is detected by a motor current detector 37 and is fed back to the subtracting section 33.
In a case that an ordinary failure of the electrical apparatus such as the electric power steering apparatus is settled, although depending on a failure region, it has been processed to settle the failure when a fail-flag to be detected with exceeding a failure threshold is continuously counted from one time to thousands times at a sampling of the detection reading out. In a case of a discontinuous counting, it is difficult to settle as a failure because of the continuous flag is not established. Further, although the discontinuous flag tends to become continuous if the failure threshold is abated, it is a problem to possibly become to a false detection.
FIG. 3 shows a characteristic example of a conventional failure detection in the prior art, and shows an example of a probability density curve in which an upper-limit side failure threshold is set to “5.35V” and a lower-limit side failure threshold is set to “4.85V” by utilizing a characteristic that a sum of output voltages normally becomes “5.10V” in a typical case. An abscissa shows the sum (diagnostic value) of the output voltages and an ordinate does the probability density.
An operation example of the failure detection in the above case is shown in a flow chart of FIG. 4. That is, the sum of the output voltages is measured in an every 1 ms (Step S1), and it is judged whether the measured value (diagnostic value) of the output voltages is equal to or less than “4.85V” of the lower-limit side failure threshold or not (Step S2). In a case that the measured value is equal to or less than “4.85V”, the counting by the fail-counter is performed (Step S4). In a case that the measured value of the output voltages exceeds “4.85V” of the lower-limit side failure threshold, it is further judged whether the measured value of the output voltages is equal to or more than “5.35V” of the upper-limit side failure threshold or not (Step S3). In a case that the measured value is equal to or more than “5.35V”, the counting by the fail-counter is performed (Step S4). In a case that the measured value is less than “5.35V”, the operations are repeated by returning to the above Step S1.
The judgement orders of the lower-limit side failure threshold and the upper-limit side failure threshold may be alternative.
At the above counting time of the fail-counter, it is judged whether the counts of the fail-counter are continuously performed 500 times or not (Step S5). In a case that the continuous counting number is less than 500 times, the operations are repeated by returning to the above Step S1, and in a case that the continuous counting number is equal to or more than 500 times, the failure is settled (Step S6).
As stated above, since the failure is settled in the prior art when the continuous counting number exceeds a predetermined number (e.g. 500 times), the discontinuous flag tends to become continuous if the upper-limit side failure threshold and the lower-limit side failure threshold are decreased. However, there is a problem to settle a false detection (misdetection) in spite of the normality
Further, as shown in a characteristic diagram of FIG. 5, the fail-counter cannot count the flags between the upper-limit side failure threshold and the lower-limit side failure threshold, it is impossible to detect a neighboring failure which does not exceed the failure threshold. That is, it is impossible to detect the intermittent failure which is a failure based on an irreversible characteristic variation, a characteristic variation due to a secular deterioration or the like.
On the other hand, as a recent requirement of vehicle electronic control devices of the electric power steering apparatus, there is a strong request to detect not only a continuous failure (permanent failure) but also the intermittent failure. It is very difficult to detect the intermittent failure and then settle. For example, if the settlement of the intermittent failure is too early, the system causes down in spite of the assist function of the electric power steering apparatus is possible to use yet and maybe a conflict phenomenon for a life extension ideology of the recent vehicle electronic control.